The present invention relates generally to ink employed in ink-jet printing. More particularly, it is directed to inks and to a system of inks in which color bleed is reduced or eliminated.
Ink jet printing is accomplished by ejecting ink from a nozzle toward paper or another print medium. The ink is driven from the nozzle toward the medium in a variety of ways. For example, in electrostatic printing, the ink is driven by an electrostatic field. Another ink jet printing procedure, known as squeeze tube, employs a piezoelectric element in the ink nozzle. Electrically-caused distortions of the piezoelectric element pump the ink through the nozzle and toward the print medium. In still another ink jet printing procedure, known as thermal or bubble ink jet printing, the ink is driven from the nozzle toward the print medium by the formation of an expanding vapor phase bubble in the nozzle. These various printing methods are described in xe2x80x9cOutput Hard Copy Devices,xe2x80x9d edited by Durbeck and Sherr, Academic Press, 1988 (see particularly chapter 13, entitled xe2x80x9cInk Jet Printingxe2x80x9d).
Preferably, an ink jet printer is capable of printing with colored ink, such as magenta, cyan and yellow, as well as black ink. When two colors are printed side by side, particularly when black ink is printed next to any other colored ink, the colors can xe2x80x9cbleedxe2x80x9d into one another. xe2x80x9cBleedxe2x80x9d is defined as the migration of one ink color into a region of another ink color, particularly when black ink moves into a region of any other color. It is desirable to have a clean, crisp border between areas of two different colors. When one color bleeds into the other color, the border becomes irregular and ragged.
Bleed is particularly undesirable when black ink is printed next to a light color ink, such as yellow.
Numerous methods have been developed in an attempt to reduce or eliminate the bleed between different colors of ink, particularly the bleed between black ink and colored ink. Common methods used to control the bleed between different colors of ink include surface tension control, chelation control, and neutralization control.
The present invention is directed to black inks and to a system of black and colored inks in which color bleed is substantially reduced or eliminated.
The cationic pigmented ink of the present invention contains at least one cationic dye as a dispersant in a pigment dispersion contained in the ink. The interaction between a stable cationic ink dispersion and a stable anionic ink dispersion is used to reduce black-to-color bleed.
Using a cationic dye as a dispersant provides several advantages over using conventional polymeric dispersants to disperse a pigment. Due to their aromatic structure, cationic dyes have a strong interaction with pigments. Also, cationic dyes are soluble in acid solutions. As a result, cationic dyes are capable of stabilizing pigments in acidic solutions. In contrast, in acid solutions the charge of conventional anionic polymeric dispersants is neutralized and the solubility of the anionic polymeric dispersant decreases. Specific cationic dyes can also change the shade and/or optical density of a pigment.
An additional advantage to using cationic dyes as a dispersant includes the resulting light fastness of the ink. Using only a cationic dye in ink jet ink formulation produces inks with poor lightfastness. When a cationic dye is used as a dispersant, the combination of a cationic dye and a pigment produces an ink jet ink with improved lightfastness.
In the present invention, the flocculation characteristics or, in other words, the charge neutralization interaction, between the cationic dispersion based pigmented ink and anionic dispersion based ink is utilized. When the cationic dispersion based pigmented ink comes into contact with the anionic dispersion based ink, flocculation occurs at the interface, thus reducing or eliminating diffusion or bleed between different colors of ink.
In a first aspect, the present invention provides a cationic pigmented dispersion comprising a cationic dye as a dispersant and a pigment. Any dye known for use in ink jet printing may be employed as a cationic dye in the practice of the present invention so long as 1) the dye has a net positive charge, 2) the dye is capable of dispersing a pigment and comprises a hydrophobic region and a hydrophilic region, and 3) the dye exhibits sufficient solubility in water to be used in ink jet ink applications. Examples of cationic cyan dyes include, but are not limited to, any of the oxazine dyes, such as Basic Blue 3, Nigrosine, and Astra Blue. Examples of cationic magenta dyes include any of the polymethine dyes, such as Basic Violet 7 and Basic Red 14 and 15. Examples of catonic yellow dyes include, but are not limited to, the yellow dyes described in U.S. Pat. No. 5,969,112. In an embodiment, the cationic pigmented dispersion comprises a cationic dye and a black pigment, wherein the cationic dye is milled with the black pigment.
In another aspect, the present invention provides an ink composition comprising a cationic pigmented dispersion and an aqueous vehicle, wherein the cationic pigmented dispersion comprises a pigment and a cationic dye.
In another aspect, the present invention provides an ink set comprising a first aqueous ink comprising a cationic pigmented dispersion comprising a cationic dye and a pigment; and a second aqueous ink comprising an anionic dispersion.
In another aspect, the present invention provides a method for producing a cationic pigmented dispersion comprising milling a mixture comprising a pigment and a cationic dye.
In another aspect, the present invention provides a method for reducing intercolor bleed on a printed substrate comprising the steps of: applying a first aqueous ink to a first area of a substrate, said first aqueous ink comprising a pigmented dispersion comprising a cationic dye and a pigment; and applying a second aqueous ink to a second area of said substrate, said second aqueous ink comprising a pigment and an anionic dispersant, wherein said first area and said second area are in communication with each other. The steps of applying a first aqueous ink and applying a second aqueous ink may occur in any order or simultaneously.